This project proposes to investigate the spinal circuitry involved in the generation of locomotion in the lamprey, a primitive vertebrate. The following questions will be addressed: 1) What spinal cells are active during fictive locomotion and what are their contributions to the locomotor network? 2) Are the synaptic potentials from locomotor interneurons altered during fictive locomotion and what factors contribute to such changes? 3) What candidate neuropeptide transmitters affect the locomotor network and how can these effects be understood by their actions on the locomotor neurons? The experiments will use an in vitro preparation of the lamprey brain and spinal cord. Simultaneous intracellular membrane potential recordings of spinal interneurons and their postsynaptic motoneurons will be made during locomotor activity induced by electrical stimulation of the spinal cord in the brain-spinal cord preparation. To determine whether or not interneurons active during locomotion have access to the rhythm generator, their ability to alter the rate of fictive locomotion in the isolated spinal cord will be tested by intracellular stimulation. The necessity in rhythm generation of interneurons with axons crossing the midline or with axons descending ipsilaterally will be tested by selective photo-ablation of these cells after retrograde filling of their cell bodies with Lucifer Yellow. The synaptic potentials that spinal premotor interneurons produce in quiescent motoneurons will be compared to those in active motoneurons to determine whether synaptic potential amplitudes and time courses change during locomotor activity. Factors that may contribute to these changes will be investigated. Such factors may include frequency of presynaptic firing, postsynaptic membrane potential, and the presence of other neurotransmitters. Finally, several neuropeptides that have recently been detected in the lamprey spinal cord with immunohistochemical techniques will be tested on fictive locomotion. These neuropeptides will also be tested for effects upon individual neurons in the locomotor network. This research will provide detailed information about how the vertebrate spinal cord generates locomotor activity. Such information may ultimately be important in the clinical treatment of spinal cord dysfunction due to injury or disease.